Slow sand filters are known to be effective in removing bacteria, parasites, Giardia cysts, Cryptosporidium oocysts and viruses. While bacteria, parasites and viruses may be killed or rendered inactive with disinfection, dormant cysts and oocysts are very resistant to disinfection unless disinfectant is administered very intensively. Slow sand filters may be used to supply clean water directly for consumption or may be used to supply water for further treatment, such as by reverse osmosis and ultra- filtration, both of which processes require quite clean water to begin with.
In the prior art, slow sand filters are known that require a continuous water supply, and that are intended for use with continuously flowing water. Such a prior art slow sand filter is described in U.S. Pat. No. 5,032,261 to Pyper. The slow sand filter includes a container partially filled with sand. The slow sand filter is provided with a water inlet and water outlet. Water continuously flows through the sand at a rate of about or slightly over 0.08 m/hour, with the sand providing a filter effect for the flowing water. In addition, when the sand is immersed in the flowing water for several days, a biolayer forms at the top of the sand. This layer is referred to as a schmutzdeuke, which is formed from various organisms such as algae, plankton, diatoms, protozoa and bacteria. The schmutzdeuke entraps and partially digests and breaks down organic matter container in the raw water passing through the filter. The organic matter accumulates on the schmutzdeuke and simple inorganic salts are formed. At the same time, inert suspended particles in the raw water are removed mechanically.
Such slow sand filters are not, however, effective where the water supply is intermittent, such as at country cottages, which limits the utility of prior art slow sand filters. In such cases, the water in prior art slow sand filters becomes stagnant and the schmutzdeuke dies.
Thus, in Visscher et al, "Slow Sand Filtration for Community Water Supply", Technical Paper no. 24, International Reference Centre for Community Water Supply and Sanitation, The Hague, The Netherlands, 1987, at page 31 it is stated that intermittent operation of slow sand filters should not be permitted because "it has been shown conclusively that an unacceptable breakthrough of bacteriological pollutants occurs four to five hours after the filters recommence operation". Likewise, Huisman, "Slow Sand Filtration", World Health Organization, Geneva, 1974, at page 32 teaches that slow sand filters should be operated at as constant a filtration rate as possible. Thus the understanding in the art was that slow sand filters should not be operated intermittently. By intermittently it is not meant that the slow sand filter be occasionally taken out of operation for maintenance, such as is described in U.S. Pat. No. 4,765,892 of Hulbert, but that the slow sand filter remains in operation, with the schmutzdeuke alive, during the period when there is no flow of water through the filter. By contrast with the present invention, the filter of Hulbert is cleaned during maintenance, so that the schmutzdeuke does not remain alive until it is operated again.
For a slow sand filter of the type described by Hulbert, with depth of water above the filter material being in the range of 60 cm to 2.44 m, the schmutzdeuke will begin to die immediately as the water flow is shut off during periods of intermittent operation and will die overnight due to lack of oxygen. This can be shown from theoretical calculations of the oxygen demand of the schmutzdeuke.
The inventor has discovered that by maintaining the static water level above the top of the sand (or other filter material) above a minimum depth, such that the schmutzdeuke does not dry out or be disturbed by the incoming water supply, and below a maximum depth, such that the schmutzdeuke is not killed by stagnant water, the schmutzdeuke may be preserved and the effectiveness of the slow sand filter maintained even where the supply of water is intermittent. The static water level, or maintenance level, is the water level in the slow sand filter while it is not being operated, that is, not being used to supply water. The schmutzdeuke layer of the slow sand filter of the present invention will typically survive more than a weekend and may survive more than a week in conditions of no flow of water between periods of flow of water through the filter, such as may occur while a cottager is away from a country cottage, and ultimately the schmutzdeuke is not killed off by a lack of oxygen but by a lack of food. By providing a life for the schmutzdeuke greater than 24 hours during periods when there is no flow of water through the slow sand filter, the present invention is believed to be distinguished from the prior art.
This is a surprising result in that it was considered in the art that the schutzdeucke would die in slow sand filters in which water was not flowing. The present invention therefore provides a substantial improvement over the known prior art.
Since normal operation of a slow sand filter results in the biological layer increasing in thickness, with a consequent reduction of flow of water, intermittent operation of the slow sand filter of the present invention actually improves flow due to the slight starvation and size reduction of the biological layer.
One such intermittently operating slow sand filter uses the damming of water in a raised pipe to provide a constant maintenance level of water in a slow sand filter container. The container includes a water inlet and outlet, and is partially filled with sand or other filter material. A pipe from the water outlet in the slow sand filter is raised upward to the desired level of water over the sand before descending to a water reservoir. Water backs up in the pipe and the container to the desired level above the top of the sand, even when the flow of water is intermittent. The schmutzdeuke forms at the top of the filter medium. The water level in such a manually operated slow sand filter, using fine quartz sand as the filter medium, and at ambient temperatures (about 21.degree. C.) is maintained above about 1 cm above the top of the filter medium and below about 8 cm above the top of the slow sand filter.
In another such intermittently operating slow sand filter, the water level is maintained automatically when not being used at a desired maintenance level, and otherwise is kept in a desired range of levels above the top of the sand (or other filter material) in a partially filled container. Automatic control is provided by a control system that senses the water level in the slow sand filter. The supply of water may be from a pump or from a gravity flow controlled by a valve. During times when water is not being taken from the slow sand filter, the water level is maintained at a desired maintenance level, within a range such as specified above for the manually operated slow sand filter.
The water from an automatic intermittently operating slow sand filter is preferably supplied to a reservoir. Water level in the reservoir may also be controlled within a desired range using upper and lower level limit detectors and a valve on an outlet line from the container to the reservoir.